1. Field of the Invention
The present invention relates to an etching method for semiconductor wafers, particularly a method for use in etching a semiconductor substrate on which a fine pattern exists, and to an etching apparatus.
2. Description of the Related Art
When a semiconductor device is manufactured, a silicon nitride film (SiN film) is variously used in a step of forming semiconductor devices such as an integrated circuit in a semiconductor device forming region of a semiconductor wafer (hereinafter referred to as a wafer). For example, the film is used as a mask material for use in forming a deep trench to form capacitors in a device forming region of the wafer including a silicon semiconductor substrate. The film is also used as an insulating film which protects gate electrode materials such as polysilicon in patterning a gate electrode, and as a protective film which protects the back surface of the silicon semiconductor substrate from corrosion, damage, and the like during various processes.
Acids such as phosphoric acid are usually used to etch/remove the silicon nitride film (SiN film) deposited on a semiconductor substrate. When phosphoric acid is used, H3PO4 is charged as an etchant at about 140 to 170° C. in an etching bath constituting an etching apparatus, the wafer is charged in the bath to perform etching. Thereafter, pure water cleaning is carried out, and drying is then carried out to end the etching.
A wet etching method and apparatus using phosphoric acid are disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 11-168081. In the etching apparatus of this document, a plurality of silicon wafers are charged in the etching bath filled with an etchant formed of an H3PO4 solution. The etching bath is connected to a discharge pipe and supply pipe, and these pipes are connected to a circulation system of the etchant, including a pump, filter, line heater, and the like.
The etchant in the etching bath is circulated in the circulation system by the pump, and impurities are removed by the filter. Furthermore, the etchant is heated to the predetermined temperature by the line hater, and returned as clean etchant again to the etching bath.
The solvent of the H3PO4 solution, that is, water easily evaporates. Therefore, pure water is appropriately replenished to keep the etchant at a constant temperature of 155° C. or more. Furthermore, in this apparatus, the top surface of the etching bath is opened in an initial stage of heating, droplets are prevented from dropping into the etchant due to condensation, and this prevents the droplets from being boiled or the temperature of the etchant from dropping. Therefore, the etchant heating can be quickly completed.
On the other hand, a method of managing a concentration of a phosphoric acid etchant is described, for example, in Jpn. Pat. Appln. KOKAI Publication No. 9-129588, and an example of a phosphoric acid concentration management system by measurement of electric conductivity is described.
Moreover, in Jpn. Pat. Appln. KOKAI Publication No. 2001-23952, in order to prevent silicon from being dissolved from the semiconductor wafer and to prevent the etching rate from fluctuating, a method is disclosed in which a predetermined amount of etchant is discharged, when a silicon concentration reaches a certain value, and a new etchant is replenished to control the etching rate. In this technique, the etchant is maintained in a boiled state at a predetermined temperature to keep the phosphoric acid concentration in the etchant substantially constant.
Furthermore, in Jpn. Pat. Appln. KOKAI Publication No. 11-145107, a technique is disclosed in which the etchant is constantly kept in the boiled state, and the pure water is replenished so as to hold the etchant at the predetermined temperature.
Additionally, in Jpn. Pat. Appln. KOKAI Publication No. 7-230981, a method is described in which a change of the concentration of the phosphoric acid etchant is sensed by a fluctuation in the number of strokes of a stroke pump, and an aqueous phosphoric acid solution is replenished to manage the concentration of the etchant. At this time, the solution is kept at 150° C. by a temperature adjustment unit.
Moreover, it is known that the phosphoric acid etching for the silicon nitride film (SiN) is sensitive at the temperature of H3PO4. Therefore, when the wafer is charged in the etching bath, the temperature of the H3PO4 etchant fluctuates. Furthermore, the fluctuation width also changes with the number of wafers per batch to be charged. Therefore, there is a problem that the SiN etching amount fluctuates, when a certain treatment is performed for an etching time.
Therefore, there has been a demand for realization of an etching method in which the etching is possible with an even etching amount of the SiN film on the wafer and with high precision among batches or in the batch, and an etching apparatus for carrying out the method.